3.5 Aircraft Wiring, Bonding & Circuit Protection Devices

3.5 Aircraft Wiring, Bonding & Circuit Protection Devices

Aircraft electrical wiring constitutes a critical physical subsystem subject to intense environmental, mechanical, and electrical stresses. Aviation Maintenance Technicians must adhere strictly to FAA Advisory Circular AC 43.13-1B Chapter 11 (Aircraft Electrical Systems) and 14 CFR Parts 23, 25, 43, and 91 when sizing conductors, routing wire bundles, selecting circuit protection, and inspecting grounding systems.

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1. Aircraft Conductor Metallurgy & AWG Sizing Standards

Conductor Materials

• Stranded Copper: The universal standard for aircraft airframe wiring (MIL-W-22759 / SAE AS22759). Conductors use fine multi-strand twisting (typically 19 to 133 individual strands) to resist vibration fatigue and flexing fracture. To prevent copper oxidation, individual strands are plated with:
  • Tin: Rated up to $150^\circ\text{C}$ ($302^\circ\text{F}$).
  • Silver: Rated up to $200^\circ\text{C}$ ($392^\circ\text{F}$).
  • Nickel: Rated up to $260^\circ\text{C}$ ($500^\circ\text{F}$).
• Aluminum Conductors: Used primarily for large power feeder runs (AWG 8 and larger) where substantial weight savings justify increased maintenance vigilance. Aluminum wire requires special installation precautions:
  • Prone to cold creep/flow under screw terminal pressure, leading to loose connections.
  • Forms an insulating aluminum oxide surface film upon exposure to air, requiring zinc-chromate or anti-oxidant joint compound (e.g., Alnox) during termination.
  • Highly susceptible to galvanic corrosion when coupled to dissimilar metals.

Wire Insulation Materials & Polyimide Cautions

• ETFE (Tefzel) & PTFE (Teflon): Modern industry standard insulations offering excellent dielectric strength, abrasion resistance, chemical resistance to aviation fuels and Skydrol hydraulic fluid, and wide temperature ratings ($-65^\circ\text{C}$ to $+200^\circ\text{C}$). Composite insulation constructions (such as PTFE/polyimide/PTFE) combine toughness with high dielectric performance.
• Polyimide (Kapton) Cautions: First-generation aromatic polyimide tape insulation was widely installed on transport aircraft in the 1970s and 1980s. Under exposure to moisture, mechanical chafing, and alkaline fluids, old Kapton insulation is prone to hydrolysis and dry/wet arc tracking—a catastrophic failure mode where an electrical arc carbonizes the insulation into a conductive graphite path, causing explosive electrical flashovers along the entire wire bundle. FAA Aging Airplane rules and Airworthiness Directives prohibit installing unjacketed aromatic polyimide wire in severe wind and moisture environments (SWAMP areas) or high-vibration engine zones.

American Wire Gauge (AWG) Numbering System

The AWG system uses an inverse numerical scale: smaller numerical gauge numbers correspond to larger wire diameters, larger cross-sectional areas, and higher current-carrying capacities (e.g., AWG 4 is much larger than AWG 20).

AWG Size vs Conductor Diameter Comparison:
  AWG 22 (Small):    (•)   d = 0.0253 in, Area = 640 CM
  AWG 16 (Medium):  ( • )  d = 0.0508 in, Area = 2,580 CM
  AWG 8  (Large):  (  •  ) d = 0.1285 in, Area = 16,510 CM

• Unit of Area (Circular Mil): Area of a circle having a diameter of $1\text{ mil}$ ($0.001\text{ inch}$): $\text{Area in Circular Mils (CM)} = [\text{Diameter in Mils } (d)]^2$
• Minimum Airframe Gauge Rules (AC 43.13-1B):
  • AWG 20 is the minimum wire size permitted for general airframe wiring runs to ensure sufficient mechanical strength.
  • AWG 22 is permitted only within securely tied and clamped bundles.
  • AWG 24 is restricted to shielded instrument harnesses and electronic equipment enclosures.

Allowable Voltage Drop Limits per AC 43.13-1B

• Conductor Sizing Equation (by Voltage Drop): $\text{Circular Mil Area (CM)} = \frac{\rho \times I \times L}{V_{drop}}$
  • $\rho = 10.79,\Omega\cdot\text{CM/ft}$ for copper at operating temperature ($17.0$ for aluminum).
  • $I = \text{Load Current in Amperes}$.
  • $L = \text{One-way Conductor Run Length in Feet}$.
  • $V_{drop} = \text{Maximum Allowable Voltage Drop in Volts}$.

2. Wire Bundling, Routing, Clamping & Inspection Practices

Clamping and Support Standards (AC 43.13-1B Chapter 11)

• Cushion Clamps: Wire bundles must be secured using MS21919 rubber- or fluorosilicone-cushioned metal clamps. Bare metal clamps or unlined plastic ties that can chafe insulation are prohibited. Clamps must hold the bundle firmly without pinching or deforming wire insulation.
• Clamp Spacing: Clamps must be spaced not more than $12\text{ to }15\text{ inches}$ apart along the run.
• Bundle Sag / Deflection Limit: Wire bundles must have sufficient slack for component servicing and terminal strain relief, but maximum bundle deflection (sag) between support clamps must not exceed $3/8\text{ inch}$ ($0.375\text{ in} / 9.5\text{ mm}$) under moderate hand pressure.
• Minimum Bend Radius: The internal bend radius of a wire bundle must be at least $10\text{ times}$ the outside diameter ($10\times\text{OD}$) of the bundle. For single wires or rigidly clamped breakouts, minimum bend radius is $3\times\text{OD}$.
• Service Loops: A service loop (extra wire length of 2 to 3 inches) must be provided at terminal junction strips and electrical connectors to allow for at least two to three future terminal re-terminations or pin replacements during maintenance.

Wire Bundle Clamp & Sag Geometry:
  [MS21919 Clamp]================ Wire Bundle ================[MS21919 Clamp]
                                  \          /
                                   \        /
                                    v      v
                                   3/8" (0.375 in) Max Sag

Drip Loops and Liquid Ingress Protection

• A Drip Loop is a downward loop formed in a wire bundle immediately before entering an electrical connector, terminal junction box, or bulkhead penetration.
• Moisture and condensed liquids running along the bundle collect at the lowest point of the loop and drip harmlessly away from the connector pins, preventing pin short circuits and galvanic terminal corrosion.

Separation from Flammable Fluid, Oxygen, and Hydraulic Lines

• Vertical Hierarchy: Electrical wiring must ALWAYS be routed ABOVE flammable fluid, fuel, hydraulic, or oxygen plumbing to ensure leaking fluids cannot drip onto electrical conductors.
• Clearance Distances:
  • Standard Minimum Separation: At least $6\text{ inches}$ ($150\text{ mm}$) of physical clearance must be maintained between wire bundles and combustible fluid plumbing.
  • Constrained Airframe Exception: In cramped structural locations where $6\text{ inches}$ is physically impossible, a minimum clearance of $2\text{ inches}$ ($50\text{ mm}$) is permitted, provided the wire bundle is rigidly clamped to primary structure so it cannot deflect, has no splices, and contains no electrical connectors in that zone.

Conduit Sizing & Fill Factor

• When pulling wire bundles through rigid aluminum or flexible conduit, the inside cross-sectional area of the conduit must be at least $25%$ larger than the combined cross-sectional area of the wire bundle ($75%$ maximum conduit fill factor).
• Drain holes ($1/8\text{ inch}$) must be drilled at the lowest points of conduit runs to prevent water accumulation.

3. Circuit Protection Devices: Fuses & Trip-Free Circuit Breakers

FUNDAMENTAL AIRWORTHINESS PRINCIPLE: Purpose of Circuit Protection

Circuit protection devices (fuses, thermal breakers, magnetic breakers) are selected and sized EXCLUSIVELY TO PROTECT THE AIRCRAFT WIRING FROM OVERHEATING, INSULATION MELTDOWN, AND FIRE—they are NOT designed to protect the connected end-load appliance!

• The current rating of a fuse or circuit breaker must never exceed the maximum allowable current capacity of the smallest gauge wire in the protected circuit.

Trip-Free Breaker Normal State:        Trip-Free Overload (Plunger Held Down):
       [ Plunger ]                            [ Plunger HELD Down ]
           ||                                        ||
    ---o---/\---o--- (Contacts Closed)        ---o---  /  ---o--- (Contacts OPEN)

Trip-Free Circuit Breaker Mandate (14 CFR 23.1357 & 25.1357)

• FAA Regulatory Mandate: All circuit breakers installed in essential aircraft electrical systems must be of the "Trip-Free" design.
• Definition of Trip-Free: A trip-free circuit breaker will open the circuit and separate its internal electrical contacts during an overcurrent fault EVEN IF THE OPERATING PLUNGER / RESET BUTTON IS MANUALLY HELD IN THE "ON" / DEPRESSED POSITION by the flight crew or maintenance technician.
• Non-trip-free circuit breakers (which can be held closed manually or reset automatically) are strictly prohibited in aircraft flight circuits because holding a tripped breaker closed during a continuous short circuit will instantly ignite wire insulation and structural airframe fires.

In-Flight Circuit Breaker Resetting Protocol

• If a circuit breaker trips in flight, FAA standard airmanship and operating handbooks dictate: Allow a minimum cooling period of 2 to 3 minutes, then attempt ONE SINGLE RESET ONLY if the system is essential to flight safety.
• If the breaker trips a second time, NEVER attempt another reset. Leave the breaker tripped, record the discrepancy in the aircraft maintenance log, and troubleshoot on the ground.

Aircraft Spare Fuse Inventory Rules (14 CFR 91.205(c)(6))

For aircraft operated at night (VFR Night) or under Instrument Flight Rules (IFR) equipped with replaceable cartridge fuses:

• The aircraft must carry a spare fuse kit containing at least one spare fuse of each rating, or $50%$ of the total number of fuses of each rating installed, whichever is greater.
• Example: If an aircraft panel contains eight $5\text{A}$ fuses and two $15\text{A}$ fuses, it must carry four $5\text{A}$ spares ($50%$ of 8) and one $15\text{A}$ spare ($50%$ of 2 = 1, minimum 1).

4. Switch Derating Factors and Load Calculations

Electrical switches are rated by manufacturers for continuous, non-inductive direct current resistive loads. When switches control non-resistive loads, high transient inrush currents or inductive kickback arcs require Switch Derating:

Standard Switch Derating Factors per AC 43.13-1B

Switch Orientation Standard: Two-position toggle switches must be mounted in aircraft panels so that the toggle handle moves UP or FORWARD for the "ON" position, and DOWN or AFT for the "OFF" position.

5. Electrical Bonding, Grounding & Static Discharge

Grounding vs. Bonding

• Grounding: Connecting an electrical return conductor directly to the metallic airframe structure, establishing a single-wire ground return path to save conductor weight.
• Bonding: Mechanically connecting all isolated structural metal components (control surfaces, engine cowlings, access doors) with low-resistance flexible metallic braided straps (bonding jumpers).

Bonding Jumper Requirements and Resistance Limits

• Materials: Tinned copper braid or aluminum alloy jumpers. Copper jumpers must not be attached directly to aluminum structure without cadmium-plated washers to prevent galvanic corrosion.
• Airworthiness Resistance Limit: The total electrical resistance across any structural bonding jumper, control surface hinge bond, or static ground joint must NOT EXCEED $0.003\text{ Ohms}$ ($3\text{ milliohms} / 3\text{ m}\Omega$).
• Purposes of Bonding:
  1. Lightning Strike Protection: Conducts lightning strike currents (up to $200,000\text{ Amperes}$) safely through the outer skin to exit points without arcing across hinges or penetrating fuel vapor spaces.
  2. Radio Shielding / EMI Suppression: Prevents high-frequency radio frequency interference (RFI).
  3. Electrostatic Grounding: Dissipates friction-induced static charges.

Static Discharge Wicks

• Precipitation Static (P-Static): When an aircraft flies through precipitation (rain, snow, ice crystals, or dust), frictional contact strips electrons from the airframe, accumulating high static voltage ($>100,000\text{V}$). This charge discharges into the atmosphere from sharp structural points in noisy, uncontrolled radio-frequency corona bursts, causing complete loss of VHF communication and ADF navigation.
• Static Dischargers (Wicks): Resistive graphite-impregnated fibrous wicks installed on trailing edges of ailerons, elevators, rudder, and wingtips. They bleed accumulated static charge smoothly and continuously into the trailing airflow at low energy levels, eliminating radio interference.

6. Worked Numerical Examples

Example 1: Conductor Sizing by Voltage Drop Formula

Problem: A $28.0\text{V}$ DC continuous landing light circuit draws $15.0\text{ Amperes}$ over a one-way wire run length of $40.0\text{ feet}$. Per AC 43.13-1B, maximum allowable voltage drop under continuous load is $1.0\text{ Volt}$. Annealed copper has $\rho = 10.79,\Omega\cdot\text{CM/ft}$.

• Calculate the minimum required Circular Mil (CM) area and select the appropriate standard AWG wire size.

Standard AWG Wire Table:

• AWG 16: $2,580\text{ CM}$
• AWG 14: $4,110\text{ CM}$
• AWG 12: $6,530\text{ CM}$
• AWG 10: $10,380\text{ CM}$

Solution:

1. Calculate required Circular Mil area: $\text{Area (CM)} = \frac{\rho \times I \times L}{V_{drop}} = \frac{10.79 \times 15.0\text{ A} \times 40.0\text{ ft}}{1.0\text{ V}} = \frac{6,474.0}{1.0} = 6,474\text{ Circular Mils}$
2. Compare with standard AWG table:

• AWG 14 ($4,110\text{ CM}$) is too small ($4,110 < 6,474$).
• AWG 12 ($6,530\text{ CM}$) meets and exceeds the minimum requirement ($6,530 > 6,474$).
• Conclusion: Install AWG 12 MIL-W-22759 copper wire.

Example 2: Switch Derating for an Inductive Solenoid Load

Problem: A hydraulic shutoff solenoid valve on a $28.0\text{V}$ DC system draws a continuous coil current of $4.0\text{ Amperes}$.

• Calculate the minimum nominal switch current rating required per AC 43.13-1B inductive derating standards.

Solution:

1. Identify the derating factor for inductive loads: $0.50$ (or multiplier $2.0\times$): $\text{Minimum Switch Rating} = \frac{\text{Continuous Load Current}}{\text{Derating Factor}} = \frac{4.0\text{ A}}{0.50} = 8.0\text{ Amperes}$ $\text{Or: } \text{Rating} = 4.0\text{ A} \times 2.0 = 8.0\text{ Amperes}$
2. Conclusion: Select a switch with a nominal rating of at least $8.0\text{ Amperes}$ (or the next higher standard rating, such as a $10\text{A}$ switch).

Example 3: Aircraft Spare Fuse Inventory (14 CFR 91.205)

Problem: An aircraft electrical fuse panel contains:

• Twelve $5\text{A}$ fuses
• Six $10\text{A}$ fuses
• Two $20\text{A}$ fuses
• One $30\text{A}$ fuse
• Calculate the required spare fuse kit inventory for VFR Night / IFR operations.

Solution:

1. Calculate $50%$ count for each rating (minimum 1 of each rating):

• $5\text{A}$ fuses: $12 \times 0.50 = 6\text{ spare fuses}$
• $10\text{A}$ fuses: $6 \times 0.50 = 3\text{ spare fuses}$
• $20\text{A}$ fuses: $2 \times 0.50 = 1\text{ spare fuse}$
• $30\text{A}$ fuses: $1 \times 0.50 = 0.5 \implies 1\text{ spare fuse}$ (minimum 1 required)

1. Total spare fuses required onboard: $6 + 3 + 1 + 1 = 11\text{ spare fuses}$.